It has already been proposed in Austrian Pat. No. 345 579, the British Pat. No. 1,192,927 and U.S. Pat. No. 3,214,565 in particular, to form a heating body from a metal sheet by providing a conductor having a substantially constant section as a result of parallel slots extending alternately from two opposite edges of this sheet and terminating at a certain distance from the edge opposite to that from which they extend in order to distribute the heating current over the entire surface of the metal sheet in a substantially uniform manner. These slots are obtained in a machining operation during which material from the metal sheet is removed over a certain width in particular by milling or sawing, in order to ensure the desired current distribution.
The manufacture of a heating body of this type requires, in particular if the metal sheet is thin, the initial fastening of this sheet to a support before the slots are cut by milling or sawing. The main drawback of the solutions proposed up to now is that the cutting of the slots requires a removal of material in order to provide a spacing between the edges of these slots thus involving a machining operation which is relatively slow and therefore badly adapted to the low cost production of a mass produced article.
It has also been proposed in the European Patent EP 0 004 829 to associate a heating body with a foam of a thermo-formable material so as to be able to mold this foam, from which a blank designed to provide an accurate fit between a piece of clothing or an accessory and a part of the human body has in the first instance been produced. This fit is obtained by moulding this blank, heated by the heating body which is incorporated therein, to the exact dimensions of this part of the human body.
This heating body is constructed using printed circuit techniques. Although this technique is completely suited to the nature of the plastic foam support, to the extent in which it enables the separate production of the heating body and its combination with the foam support, this involves a technology reserved for the manufacture of electrical circuits. The cost of manufacture of these circuits is disproportionate when this technology is only used to provide simple heating bodies designed for mass produced articles since a heating body of this type does not require the accuracy of printed circuits.
In addition it has already been proposed in the Swiss Pat. No. 421 330 to provide a heating body of the type mentioned above by stamping of a thin metal sheet glued to an insulating support. It should however be noted that a method of manufacture of this type gives rise to considerable practical difficulties, given that when the metal sheet and the insulating coating are cut, the sheet and the coating are transformed into a thin zigzag shaped strip such that the geometry of the sheet no longer exists in the absence of a support, which leads to problems in the manufacture, handling and use of the heating body.